1. Field of the Invention
The present invention relates generally to aircraft flight path design, and more particularly to cursor-aided manipulation of flight plans in flight displays.
2. Description of the Related Art
There are two basic sets of rules for flight operations, Visual Flight Rules (VFR) and Instrument Flight Rules (IFR). Visual Meteorological Conditions (VMC) are those weather conditions in which pilots have sufficient visibility to maintain visual separation from terrain, obstacles, and other aircraft. Instrument Meteorological Conditions (IMC) are those weather conditions in which pilots cannot maintain visual separation from terrain, obstacles, and other aircraft.
Under Visual Flight Rules (VFR), the pilot maintains separation from terrain, obstacles, and other aircraft by visual reference to the environment surrounding the aircraft. The guiding principle for VFR is “See and Avoid”. Under Instrument Flight Rules (IFR), the pilot maintains separation from terrain and obstacles by reference to aircraft instruments only. The guiding principle for IFR is “Positive Course Guidance” to track a “hazard-free path” which provides separation from terrain and obstacles. Separation from other aircraft is provided by Air Traffic Control. VFR principles may only be used under VMC; however, IFR principles are used under both VMC and IMC.
A key aspect of operating in IMC is determining the accuracy of the aircraft navigation systems and the performance of the pilot or automated flight systems. All navigation systems have an uncertainty in their ability to determine the position of the aircraft. The magnitude of the uncertainty is driven by the underlying technologies that are used to implement the navigation system. All pilots and automated flight systems have limitations in their ability to track an intended flight path. These limitations result in deviations between the actual position of the aircraft versus the intended position of the aircraft. This deviation in aircraft position is known as flight technical error (“FTE”). The total system error (“TSE”) is the combination of uncertainty in the navigation system and the flight technical error of the pilot or automated flight system. IFR operating procedures are designed to accommodate the TSE. The greater the possible TSE, the larger the buffer must be between the intended path of the aircraft and terrain, obstacles, or other potential hazards to the aircraft.
The simplest form of IFR operations is dead-reckoning where the pilot navigates using only magnetic heading, airspeed, and time. This allows the pilot to estimate his/her location by using a map to identify a starting point then using heading, speed, and time to determine distance and direction traveled from the starting point. Dead-reckoning is highly inaccurate in windy conditions because the pilot cannot accurately determine the actual ground speed or aircraft track (which differ from airspeed and heading due to the velocity of the wind). Modern inertial navigation systems automate the dead-reckoning process and provide much higher accuracies than the pilot can achieve without assistance. However, even the best, and most expensive, inertial navigation systems suffer from position errors that increase over time (typically with a drift rate of 2 nautical miles or more per hour). Thus, operating by dead-reckoning can result in a very large TSE.
Various navigational aides (NAVAIDS) have evolved over time to improve the accuracy of navigation in IMC, resulting in lower TSE. The first generation of NAVAIDS includes ground-based navigation radio systems such as VHF Omnidirectional Range (VOR), Distance Measuring Equipment (DME), and Instrument Landing System (ILS). These solutions allow an airborne radio receiver to determine either bearing to a ground-based transmitter (e.g. VOR) or distance to the transmitter (e.g. DME). The ILS is a specialized system that allows the airborne radio receiver to determine angular deviation from a specific bearing from the transmitter (Localizer) and specific descent path (Glide Slope). While these systems provide significant improvement in accuracy over inertial navigation systems, they require very expensive ground infrastructures which limit the number of locations where they may be installed.
Another disadvantage of ground-based radio positioning systems is that such systems provide less certainty of an aircraft's position the farther the aircraft is from the transmitter. Recognizing this limitation, regulators have established a set of criteria for building instrument-based navigational procedures called TERPS (Terminal Instrument Procedures) for designing approaches that recognize the limitations of the technology. TERPS employs trapezoidal obstacle identification surfaces that take into account inaccuracies in the aircraft's positional certainty. TERPS is formally defined in US FAA Order 8260.3B, along with associated documents in the 8260 series. The international equivalent of TERPS is called PANS-OPS, promulgated by the International Civil Aviation Organization (“ICAO”) (document 8168); the two combined represent virtually 100% of conventional approaches in place today. Procedures developed in accordance with the TERPS or PANS-OPS have serious limitations in that they are written using “lowest common denominator” aircraft performance expectations. The smallest general aviation aircraft and the largest transport jets all use the same procedures to depart and arrive at terrain-challenged airport in IMC regardless of the capabilities of the aircraft or aircrew.
The next generation of NAVAIDS exploits the Global Positioning System (GPS) infrastructure which was deployed by the Department of Defense. Airborne Satellite Navigation (SATNAV) receivers can calculate the current position of the aircraft to far greater accuracy than can be achieved with VOR and DME, with a lower TSE than using VOR and DME, and can provide similar performance to ILS near the runway threshold with similar TSE.
An emerging model for IFR operations defines operating procedures based upon the concept of Required Navigation Performance (RNP). Instead of defining approach and departure paths based upon the lowest accuracy of the available NAVAIDS, RNP defines the minimum performance requirements that an airborne system must achieve to use a published RNP procedure. In addition, a new paradigm is emerging that allows RNP procedures to be developed and published that assume Special Aircraft and Aircrew Requirements (SAAAR). Even though RNP-SAAAR procedures are published (and therefore public), they may only be used by aircraft operators that have been authorized in advance by the regulatory authorities. These RNP-SAAAR procedures will allow complex approach and missed approach procedures at terrain-challenged airport in IMC; however, there are hundreds of terrain-challenged airports around the world, and it will be a long time before procedures are developed and published for all the airports. In fact, it may be too expensive to develop RNP-SAAAR procedures for small airports that have very low utilization.
Thus, as discussed above, the TERPS defines the criteria for the creation of arrival procedure from top of descent through a successful landing or a missed approach. The TERPS uses the maximum allowed TSE for each type of navigation solution to define the necessary Obstacle Clearance Surface (OCS) (i.e., buffer between the aircraft and hazards) for a corresponding type of approach or missed approach (e.g., a precision approach versus a non-precision approach).
The missed-approach point is the location along the approach path that the pilot must decide to continue the landing or to go around. Precision approaches have a Decision Height (DH) where the pilot must decide to land or go around. Non-precision approaches have a Minimum Descent Altitude (MDA) (i.e. lowest published descent altitude), where the pilot must have visual reference to the airport to proceed. Decision heights range from 0 feet above the runway (Cat IIIc) to 200 feet (Cat I) while minimum descent altitude range from hundreds of feet to thousands of feet above the runway.
FIG. 1 (Prior Art) is a schematic illustration of a typical approach of the path of an aircraft from top of descent to missed approach point and then through the missed approach procedure where the minimum descent altitude is driven by terrain and obstacles along the approach path and/or the missed approach path.
FIG. 2 (Prior Art) shows the situation where terrain along the non-precision approach path requires the MDA to be substantially higher than the typical Decision Height on a precision approach.
FIG. 3 (Prior Art), FIG. 4 (Prior Art) and FIG. 5 (Prior Art) show the TERPS-required OCS relative to the approach path shown in FIG. 2. FIG. 3 shows the elevation difference between the intended path and the OCS where the elevation difference is derived from the vertical component of the maximum TSE of the navigation solution used to perform the approach. FIG. 4 shows a cross-sectional view of the OCS where the width of the OCS is derived from the horizontal component of the maximum TSE of the navigation solution used to perform the approach. The MDA is determined from the point where the terrain penetrates the OCS. In order to descend below the MDA, the pilot must have some means to avoid the terrain that is penetrating the OCS. This can be accomplished by the pilot having sufficient visibility to “see and avoid” the terrain or by using a better navigation solution that results in a lower TSE thereby reducing the distance to the OCS. Alternately, a different approach path may be used to avoid the terrain conflict entirely. This invention provides a system and method for generating an alternative path to avoid the terrain conflict. FIG. 5 shows a top-down view of the approach path and the location where terrain penetrates the OCS. FIG. 6 shows an example of a flight path that bends laterally around the terrain cell which caused the conflict with the original flight path shown in FIGS. 3, 4, and 5.
It is highly desirable to find a means to allow an aircraft to descend below published MDAs to increase the probability that the flight can proceed to a successful landing instead of the flight diverting to an alternative airport.
U.S. Pat. No. 7,302,318, entitled “Method for Implementing Required Navigational Performance Procedures” issued to D. J. Gerrity et al, discloses a method for designing an approach for a selected runway. The method includes gathering data regarding the height and location of all obstacles, natural and man-made, within an obstacle evaluation area. A preliminary approach path is laid out for the runway, including a missed approach segment, and a corresponding obstacle clearance surface is calculated. In the preferred method the OCS includes a portion underlying the desired fixed approach segment, and may be calculated using a vertical error budget approach. The OCS includes a missed approach segment that the aircraft will follow in the event the runway is not visually acquired by the time the aircraft reaches a decision altitude. A momentary descent segment extends between the first segment and the missed approach, and is calculated on physical principles to approximate the projected path of the aircraft during the transition from its location at the decision altitude to the missed approach segment. The preliminary path is then tested to insure that no obstacles penetrate the missed approach surface, and may be improved, e.g. lowering the decision altitude, by adjusting the OCS until it just touches an obstacle.